The present invention is directed generally to thin films, magnetic recording media, transducers and devices incorporating the films and, more particularly, to thin films promoting highly oriented cobalt or cobalt alloy magnetic layers for use in magnetic recording media and transducers.
There is an ever increasing demand for magnetic recording media with higher storage capacity, lower noise and lower costs. To meet this demand, recording media have been developed with increased recording densities and more well-defined grain structures that have substantially increased the storage capacity, while lowering the associated noise of the recording media. However, the rapid increases in recording densities over the last two decades, combined with the proliferation of personal computers have only served to fuel the demand for even higher storage capacity recording media having lower noise and cost.
Computational and data manipulation devices are being used in a rapidly expanding number of applications. Examples of these include supercomputers, personal desk top and portable laptop computers, file servers, personal data assistants, data collection devices, article tracking systems, video recorders, digital audio recorders, and even telephone answering machines. A common architectural feature is that they all have a central processing unit, input-output interfaces, various levels of temporary memory, and usually some form of permanent data storage device. The distinguishing characteristic of the permanent data storage device is that the information remains intact even if the electrical power is lost or removed. Data are stored on permanent data storage devices either optically or magnetically. The more commonly used data storage devices are based upon magnetic materials which are erasable and re-recordable. Common to all magnetic data storage devices are record and read transducers, a magnetic medium upon which to store the data, and a mechanism to position the medium or the transducers relative to one another.
Some of the more common permanent data storage devices include the floppy disk drive, the hard disk drive, and the magneto-optic disk drive in which data are stored in magnetic bits in segmented circular tracks. The magnetic medium is rotated and the transducers are stationary or moved radially to read or write data at a location on the medium.
Likewise, the magnetic medium is sometimes constructed as a tape or a sheet and is transported linearly while the transducers may be stationary, moved transversely across the moving medium, or even moved in a helical arc relative to the medium. Also, in the future it is conceived that very large amounts of data may be stored on physically very small formats where the medium or the transducers are moved in two dimensional Cartesian coordinates or arc motions relative to each other to access the data.
Historically, the transducers for many of the non-optical magnetic data storage systems have been inductive magnetic heads used for recording data by magnetizing the medium in a particular direction and for reading the data by detecting the direction of the magnetized medium. More recently, an inductive magnetic head is used for recording the data pattern while a magnetoresistive sensor is used for reading the data. In many of the magneto-optical storage devices an integral part of the record transducer is a component which generates a magnetic field at the medium surface while the surface is heated by using an optical source. The medium magnetization then assumes the magnetic orientation of the field generated by the record transducer when the medium cools. In some systems this orienting field is provided by an adjancent magnetic material.
Due to the physical size, efficiency and orientation of the record and read transducers the magnetic medium is generally magnetized in a preferred orientation. Hence, in almost all magnetic data storage media it is desired to orient the magnetic media in a direction to match the operational orientation of the recording and playback transducer. In addition, magnetic materials generally will magnetize more easily in a preferred orientation or orientations, along what are known as a magnetically easy axis or axes.
Magnetic properties, such as coercivity (H.sub.c), remanant magnetization (M.sub.r) and coercive squareness (S*), are crucial to the recording performance of the medium. These magnetic properties are primarily dependent on the microstructure of the film for a fixed composition. For thin film longitudinal magnetic recording media, the magnetized layer preferably has uniaxial crystalline anisotropy and a magnetization easy axis directed along the c-axis and predominately in the plane of the film (i.e, in-plane). The predominate crystallographic orientation of a layer is known as the crystallographic texture, or texture, as used herein, as opposed to the use of the term "texture" to describe the mechanical roughness of a surface. That is, a crystal having a surface and a crystallographic plane parallel to the surface would be said to have a texture described by a direction vector orthogonal to the surface. Usually, the better the in-plane c-axis orientation, the higher the coercivity of the magnetic layer used for longitudinal recording. High coercivity is required to achieve a high remanence. Likewise, for perpendicular magnetic recording media, the desired crystalline structure of the Co alloys is hexagonal close packed ("hcp") with the uniaxial anisotropy and crystalline c-axis perpendicular to the film plane.
It is generally desirable to align the magnetically easy orientation of the medium with the orientation of the transducers. By aligning the orientations of the medium and the transducers, a data bit can be recorded with a lower energy transducer field and the ability to more easily magnetize the medium provides for a more strongly magnetized portion of the medium. The combination of these two effects allows a data bit to be recorded to and read from a more localized, yet more highly magnetized, portion of the medium. In other words, by aligning the relative magnetic orientations of the transducers and the medium, increased recording densities and storage capacities can be achieved. This results in a higher performance data storage device by allowing more data to be stored in a smaller area on the media. It also results in a lower cost per data bit and possibly lower cost storage devices, as fewer components are required to build an equivalent or larger capacity storage device. In many cases it also results in a decreased access time to reach a particular piece of stored data since the physical size of the storage system is smaller.
In the rotating storage devices it is desirable that the orientation of the medium be either random parallel to or constant in relation to the circumferential direction in the plane of the medium or that the orientation be perpendicular to the medium surface. In each of these orientations the relative orientations of the magnetic medium and the transducers does not vary as the medium is rotated relative to the transducers. Variations in the relative orientations of the medium and the transducers results in variations in the recording and reading of signals, which is known as signal modulation.
For floppy disks and most hard disks the orientation is nearly random in the plane of the medium. However, rotating magnetic media often have some small degree of orientation along the record track direction due to the mechanical roughness of the substrate surface. For perpendicular magnetic media, the orientation must be well oriented perpendicular to the media plane to match the field orientation of the record and read transducers. In magneto-optical recording, the magneto-optical Faraday effect, or Kerr effect, is far larger when the light propagates parallel to the magnetization direction. Because the light is usually delivered perpendicular to the medium surface, it is desired that the magnetic orientation of the medium be the same. Likewise, for tape and sheet magnetic recording systems the preferred magnetic orientation of the media is parallel to the field orientation of the transducers.
Modern high performance magnetic media generally consist of one or more thin magnetic films supported on a substrate. The thin films are generally vacuum deposited on the substrate by various techniques such as thermal or electron beam evaporation, RF or DC diode or magnetron sputtering, ion beam deposition, laser ablation, or chemical vapor phase deposition. However, films have also been prepared by electrochemical deposition. In most magnetic recording media, such as used in hard disks, the thin film layers are polycrystalline. In most commercial magneto-optical recording media the magnetic thin film layers are composed of amorphous rare earth-transition metal alloys, but polycrystalline superlattices have also been used.
In hard disk applications, the substrate can be made from a glass, a glass ceramic, or ceramic, but is more commonly an AlMg alloy with a NiP layer which is electrolessly plated on the surface. Typically one or more non-magnetic underlayers, such as Cr, Cr with an additional alloy element X (X.dbd.C, Mg, Al, Si, Ti, V, Co, Ni, Cu, Zr, Nb, Mo, La, Ce, Mn, Nd, Gd, Tb, Dy, Er, Ta, and W), Ti, W, Mo, NiP and B2-ordered lattice structures, such as NiAl and FeAl, are deposited on the substrate prior to depositing the magnetic layers to promote a particular orientation and/or to control the grain size of the magnetic layers, which are typically composed of Co alloys.
Another factor that affects the recording performance is the grain size and grain separation in the magnetic layer. The size and separation of the individual grains not only affects the media noise and recording density achievable on the layer, but the separation also affects the achievable separation of the recorded data transitions, or signal, the degree of overlap of which induces additional media noise in the signal.
Cobalt-based alloys as opposed to pure cobalt are commonly used in longitudinal and perpendicular magnetic media for a variety of reasons. For example, non-magnetic elements such as Cr are commonly bulk doped into the magnetic film to lower the magnetization. This is especially important in perpendicular media where the demagnetization energy associated with the magnetic moment of the alloy must be less than the magneto-crystalline anisotropy energy in order for the magnetization to be oriented perpendicular to the media film plane. The same technique is used in longitudinal magnetic media to lower the flux transition demagnetization energy, resulting in a shorter flux transition length and, hence, higher recording densities. Even more importantly, however, non-magnetic elements are introduced into the Co-alloy to provide grain to grain isolation via non-ferromagnetic material diffusion to limit the magnetic exchange coupling between cobalt grains.
Generally, for very small grain sizes, the coercivity increases with increased grain size. Large grains, however, results in greater noise. There is a need to achieve high coercivities without the increase in noise associated with large grains. To achieve a low noise magnetic medium, the Co alloy thin film should have uniformly sized, small grains with grain boundaries which can magnetically isolate neighboring grains. This kind of microstructure, orientation, and crystallographic texture is normally achieved by manipulating the deposition process, by grooving the substrate surface, by varying the cobalt alloy composition or by the proper use of underlayers.
Separation of the grains of the magnetic layer and the resulting improvement in the signal to noise ratio of a recorded signal is promoted by inducing epitaxial crystalline growth of the magnetic layer grains. The grain size and orientation quality of the magnetic thin film layers are largely determined by the grain size and texture quality of the layer upon which the layer is being deposited. The degree to which a prior layer can be made to induce a texture in a second layer depends, in part, upon the relative size, or lattice spacing and the crystal structure, of the material in each layer. As might be expected, if there is a substantial variation between the crystal sizes and structure of layers the crystallographic texture will not be replicated and the layer will be deposited with either an amorphous structure and/or in an orientation of the crystal structure independent of the underlayer and representative of the lowest energy state, i.e. closest packed structure, depending upon the material.
For Co based magnetic media it has been found that Cr provides a good crystallographic texture for Co alloys, as Co grains tend to replicate the Cr grain size and the orientation is somewhat set by the underlayer quasi-epitaxial growth of the Co on the Cr crystallites. Depending upon the particular Co alloy being used as the magnetic layer, the underlayer alloy composition can be chosen to vary the atomic crystalline lattice parameter to better match the lattice spacing of the Co alloy. For example, U.S. Pat. No. 4,652,499 discloses efforts to improve the underlayer of longitudinal magnetic media by adding vanadium (V) to Cr to change its lattice constant and thereby to promote better lattice matching between the hcp Co alloys, such as CoPt or CoPtCr, and the body centered cubic ("bcc") CrV underlayer. Others have discussed similar results by additions of other similarly soluble large atomic radii materials, such as Ti and Mo, in the bcc structure of Cr. The lattice matching promotes the growth of the Co alloy into a hcp structure as opposed to a face centered cubic ("fcc") structure.
The hcp Co alloys have a high uniaxial anisotropy constant, along the hcp c-axis, which is necessary to achieve a high coercivity in the magnetic media. In rotating media applications, the Co alloy is epitaxially grown upon a random in the plane orientation of bcc Cr crystallites to prevent signal modulation. That is, the Co should be oriented with a preferred set of crystal planes parallel to the substrate surface plane. Several Cr textures are suitable to grow Co with its magnetic easy axis in or near the film plane. For example, K. Hono, B. Wong, and D. E. Laughlin, "Crystallography of Co/Cr bilayer magnetic thin films", Journal of Applied Physics 68 (9) p. 4734 (1990) describe bcc Cr underlayers that promote grain-to-grain epitaxial growth of hcp Co alloy thin films deposited on these underlayers and the orientation of the Co alloy [0002] c-axis, and, hence, the magnetic easy axis, is directly related to the crystallographic texture of the Cr alloy underlayer. The most common of these texture relationships between the Cr alloy and the Co alloy are summarized as:
quad-Crystal: Co(1011)[1210] .vertline..vertline. Cr(110)[110] or Co(1011)[1210] .vertline..vertline. Cr(110)[110] PA1 bi-Crystal: Co(1120)[0001] .vertline..vertline. Cr(002)[110] or Co(1120)[0001] .vertline..vertline. Cr(002)[110] PA1 uni-Crystal: Co(1010)[0001].vertline..vertline. Cr(112)[110]
While the Cr (001) (bi-crystal) and Cr (112) (uni-crystal) textures induce the Co alloy c-axis, [0002], into the film plane, the most easily formed Cr (110) texture results in the c-axis being inclined at .+-.28 degrees with respect to the surface. Hence, a lower coercivity results from the Co grown on the (110) Cr texture as the c-axes are not parallel to the recording plane. Also there are multiple directions that the Co c-axes can be placed upon the Cr (002) and the Cr (110) textures. Hence, upon a single (002) textured Cr grain 2 possible c-axis orientations can grow (bi-crystal) while upon a single (110) textured Cr grain 4 possible c-axis orientations of Cr can grow (quad-crystal). If these variants do coexist on single Cr alloy grains, the bi-crystals and quad-crystals then can never have all of the c-axes simultaneously parallel to the applied field and the coercivities of the grains will be decreased. On the other hand, the very uncommon uni-crystal Co(1010)//Cr(112) texture relationship only allows a single orientation upon a Cr grain and results in a higher coercivity whether the Cr grains are randomly oriented in the film plane or oriented parallel to the recording field direction.
At room temperature, or if a negative voltage bias is applied to the substrate during sputter deposition, it has been experimentally found that the Cr(110) texture tends to develop and assuming a reasonable lattice match exist between the Cr alloy and the Co alloy crystals the quad-crystal hcp Co tends to grow. Likewise, it has also been found that when the Cr is deposited at elevated temperatures a limited degree of Cr(002) is observed by x-ray diffraction and to a degree the Co bi-crystal tends to grow. However, in each of these cases there is considerable dispersion and variation in the texture of the Cr and in the resulting orientation of these magnetic easy axes of the Co magnetic layers. The desirable Cr (112) texture, which is required to obtain the uni-crystal, is not often seen except when Cr is made unsuitably thick for media and at which point the Cr crystallites are growing in all directions and the film is usually showing multiple textures similar to a polycrystalline powder.
Applicants have previously found that well textured Cr layers having a (002) orientation can be produced using a polycrystalline MgO seed layer as described in U.S. Pat. No. 5,800,931, which is incorporated herein by reference. In addition, Applicants have also shown that Cr (112) can be produced if the Cr is epitaxially grown on a (112) oriented B2 body centered cubic derivative material, such as NiAl and FeAl, as described in U.S. Pat. No. 5,693,426, which is incorporated herein by reference. It is also noted that Nakamura et al. have produced (002) and (112) Cr during crystal studies on single crystal MgO, NaCl structure (Jpn. J. Applied Physics, Vol. 32, part 2, No. 10A, L1410 (October, 1993) and Jpn. J. Appl. Phys. Vol. 34(1995) pp. 2307-2311).
Additional improvements in the structure of the magnetic layer have been found by incorporating intermediate layers between the underlayer and the magnetic layer. Also, seed layers can be incorporated between the underlayer and the substrate to provide additional control of the structure of the underlayer, control the roughness of the films, and to prevent contamination of the underlayer by the substrate contaminants. The multiple seed layers, multiple underlayers, and intermediate layers are collectively referred to herein as the underlayer structure. In addition, multiple magnetic layers that may or may not be separated by a non-ferromagnetic inner layer such as Cr or Cr alloys are sometimes employed to produce variations in the magnetic properties of the resulting film. The magnetic layers and intervening inner layers are collectively referred to herein as the magnetic layer structure. The use of multi-layered underlayer and magnetic layer structures can provide for increased control over the grain size, the grain to grain isolation and epitaxial growth of subsequent layers and the surface roughness of the magnetic layers. However, the use of additional layers will also increase the overall cost and complexity of the manufacturing process.
For perpendicular recording it is desired that the Co alloy c-axis be perpendicular to the substrate plane. This means that the Co alloy has a (0002) texture and the [0002] crystal direction is perpendicular to the film plane. It has been found that if Co is grown fairly thick that this texture naturally develops as the (0002) plane of atoms are closest packed. However, this is unsuitable for media as a large dispersion in this orientation results and the first portion of these films have random or extremely poor orientation.
Some degree of perpendicular orientation of the Co c-axis with respect to the film plane has been obtained by grain-to-grain quasi-epitaxial growth of the hcp Co alloy thin film to an oriented hcp underlayer of (0002) crystalline texture or a fcc underlayer. Ti and Ti.sub.90 Cr.sub.10at% are often cited as the best seed layers or underlayers for this purpose, although other seed layers, such as Pt, CoO and thick non-magnetic hcp CoCr.sub.35at% have been used to induce this structure. See "Compositional separation of CoCrPt/Cr films for longitudinal recording and CoCr/Ti films for perpendicular recording" IEEE Trans. Magn., Vol. 27, no. 6, part 2, pp. 4718-4720 (1991); and, IEEE Trans. Magn. Vol. 30, no. 6, pp. 4020-4022 (November 1994).
The degree of orientation must be exceptionally good for perpendicular recording as the perpendicular head field patterns have low field gradients because there is no return path for the field flux. To date there has been no commercially viable products using perpendicular thin film recording media that are comparable in terms of longitudinal thin film recording products.
Many attempts have been made to solve the difficulty of the poor perpendicular head field gradients by producing media that have a soft magnetic "keeper" layer, such as permalloy or CoZr alloys, deposited under the recording media. The soft layer is used to provide a high permeability magnetic flux return path to sharpen the perpendicular pole-head field gradients, which sharpens the data transitions in the recorded patterns. However, the soft magnetic layers add complexity to the media and commonly increase the media noise due to their imperfect or lack of quality and the resulting domain wall motion Barkhausen phenomena.
Magnetic recording and playback transducers are composed of soft magnetic materials such as CoZr, FeN.sub.x, FeAlN.sub.x, FeTaN.sub.x, FeSi alloys, NiFe alloys or FeCo alloys. Analogous to the media in order to produce high performance transducers it is necessary to control the crystallographic orientation, magnetic anisotropy magnitude and orientation, and grain size, as well as, magnetostrictive and magnetoelastic properties, and localized stresses in the material. Without control of these attributes and factors the transducer may have undesirably large hysteretic properties, remain magnetized even after the drive signal has been removed, exhibit Barkhausen phenomena or time delayed noise spikes, or exhibit non-linear response to field signals.
In order to produce more uniform response in fcc Ni alloy soft materials, vacuum deposited seed layers are sometimes employed to induce the fcc Ni alloy to have a (111) texture. For materials such as NiFe alloys the easy magnetic axes lie along the &lt;111&gt; directions so if a (111) texture is induced the easy axes to lie only 19.degree. from the film plane. The quality of this orientation plays a significant role in determining the uniformity of the magnetic spin rotation or the magnetic domain wall motion in these layers.
Likewise, in magnetoresistive or spin valve sensors it is common to use hard magnetic materials, very similar to those used in magnetic recording media, to provide magnetic biasing to the soft magnetic materials. For example U.S. Pat. No. 4,902,583 describes the use of CoPt for this purpose. As with magnetic media it is desirable to control the texture quality in order to improve the anisotropy and the coercivity of these device elements. In order to improve the performance of magnetic data storage transducers there is considerable need to develop methods, materials and thin film device structures which will yield a high degree of orientation and uniformity of the magnetic film properties.
As is obvious from the preceding discussions, there is a continuing need for lighter, smaller and better performing and less costly memory devices. In order to meet to this need, the underlayers must exhibit an unusually high degree of crystallographic orientation, which will then result in high degree of magnetic orientation. These devices must provide greater storage density and higher recording and reading quality and efficiency for use in today's hard disk drives, for transducers with other magnetic storage devices, and other applications.